If in the Xcode's debug console I type
(lldb) p (float)fabsf(-5.0f)
(float) $22 = 0
(lldb) p (double)fabsf(-5.0f)
(double) $23 = 5
extern float fabsf(float);
extern double fabs(double);
extern long double fabsl(long double);
It was pretty sure a bug of lldb console in Xcode 7, and a ticket was created for it.
The issue is not present any more in Xcode 8.0.